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Appendix VI.                            1151

dial valves N and L, are placed on both gas and water admissions,
and the thermometers p and Q shew the temperatures of the in-
going and outgoing water respectively. Water being turned on at
N, and gas at j, the former is at first delivered into the funnel T,
by deflecting the india-rubber tube s into the position s^ When the
apparatus is warmed and in steady working order, the test begins.
Then the pipe s runs into the beaker to be there measured, the con-
sumption of gas is noted, and the temperatures taken both at p
and Q. When the beaker u is nearly full the test is stopped, and
the heat units given to the water reckoned for the gas consumed,
being finally reduced to the calorific value per cubic foot of gas.

Pp. 705 and p/j.    Gas Engine Improvements :

Fuel.   Some remarks have already been made at p. 913 on the

various kinds of gases that can be burnt with air in the gas engine.

Some additional forms may be noted, giving a total choice of

1.  Lighting Gas : for small and medium-sized engines.

2.  Pressure-Producer Gas : )

3.  Suction-Producer Gas:    | of like composition.

4.  Water Gas : not now made intermittently, but as at 2 and 3.

5.  Mond Gas : a producer gas containing much free hydrogen.

6.  Blast-furnace Gas : consisting largely of CO.

7.  Natural Gas : found in the oil-bearing regions of America
and the Caucasus*

Lighting Gas is too expensive for very large engines. Its
principal advantage lies in the use of smaller engines for equal
power as compared with poor gas, but the thermal efficiency is not
increased, while the cost undoubtedly is.

Producer Gas i$ formed by passing steam over incandescent
carbon. Formerly practised by the intermittent method described
on j>. 915, it was called Water Gas, but the inconvenience of the
' blow? and c glow ' action led to the invention of the continuous
production of what has been sometimes called * Semi-Water Gas,7
better known in England as Dowson Gas. The same reactions
no doubt take place as with true water gas, viz. :  '

C + CO2     =   aCO
C + H26    =